Speeding motorists using Dublin's Port Tunnel will be caught using a new safety system which tracks their average speed through the urban motorway.

It was announced last June by Transport Infrastructure Ireland (TII) and An Garda Siochana that a new average speed detection system would be installed in the tunnel which would track drivers and calculate their speed through its 4.5km distance.

The system is not due to go live until the end of next month, but a TII spokesman has said the camera system which is now visible to motorists has already had an effect on driver behaviour. "It seems to be acting as a visual deterrent, because we can see on cameras that driver habits have changed," the TII told the Herald.

"People are slowing down and leaving a greater distance between them and the vehicle in front.

"There is also less lane switching, and the overall effect is safer driving, so we can only assume that will improve again when the speed detection system is switched on once testing has been completed."

Once implemented, any motorist completing the journey in anything less than three minutes and thirty seconds would be breaking the 80km/h speed limit.

When the system determines that a vehicle has exceeded the speed limit, it will automatically create a record of the violation which will then be transmitted to gardai.

This will result in automatic fines of €80 plus three penalty points.

Average speed is recorded between two camera positions covering all lanes, therefore switching lanes will have no effect on average speed monitoring.

The system will not have flashing lights or other operational indications, but it will be monitoring at all times.

The 4.5km distance is calculated from the positions of cameras positioned at the mouth of the tunnel at each end.

Detection

Once operational it will be the first average-speed camera -enforcement system in Ireland.

Around 24,000 vehicles use the tunnel every day, and research suggests that use of the average-speed detection system can reduce collision rates by as much as 50pc.

"The cameras and software have all been installed and we are now testing the system with our own vehicles to ensure its accuracy, and once testing is complete, which should be in the next few weeks, the system will go live," said the TII.